Case hardened steel with high tempering temperature, method for obtaining same and parts formed with said steel

ABSTRACT

The invention discloses a carburizing steel composition comprising, by weight: 
     0.06% to 0.18% of C; 
     0.5% to 1.5% of Si; 
     0.2% to 1.5% of Cr; 
     1% to 3.5% of Ni; 
     1.1% to 3.5% of Mo; 
     and, if appropriate: 
     at most 1.6% of Mn; and/or 
     at most 0.4% of V; and/or 
     at most 2% of Cu; and/or 
     at most 4% of Co; 
     the complement being constituted by iron and residual impurities; 
     the amounts of Ni, Mn, Cu, Co, Cr, Mo and V in said composition, expressed by weight, satisfying the following relationships: 
     
       
         2.5≦Ni+Mn+1.5Cu+0.5Co≦5  (1) 
       
     
     
       
         2.4≦Cr+Mo+V≦3.7  (2) 
       
     
     and a process for producing carburized and treated parts produced from the compositions.

CROSS-REFERENCE TO RELATED APPLICATION

This is the national phase of PCT Application No. PCT/FR99/01543,International Filing Date of Jun. 28, 1999, published in French.

The present invention relates to a carburizing steel composition, toparts formed from said steel, and to a process for producing partsformed from said steel.

Carburizing is a thermochemical surface treatment which generallyproduces parts combining good core ductility with a “case-hardened”carburized surface that is hard and resistant to wear.

Many applications require a steel with a good resistance to softening atworking temperatures. Examples that can be cited are gear wheels,bearings and transmission shafts for helicopters or for vehicles formotor racing, gear wheels, camshafts and other parts used in enginedistribution systems, fuel injectors and compressors.

The following particular carburizing steels are routinely used for suchapplications: 17CrNiMo6, 16NiCr6, 14NiCr12, 10NiCrMo13, 16NiCrMo13 or17NiCrMo17. Such steels can be used up to working temperatures of closeto 130° C., but the carburized layer has neither a resistance tosoftening nor an elevated temperature hardness sufficient for workingtemperatures exceeding 190° C.

U.S. Pat. No. 3,713,905, granted to T. V. Philip and R. L. Vedder onJan. 30^(th), 1973, describes the properties obtained for a steel withthe following chemical composition as a percentage by weight:

0.07%-0.8% of C;

at most 1% of Mn;

0.5%-2% of Si;

0.5%-1.5% of Cr;

2%-5% of Ni;

0.65%-4% of Cu;

0.25%-1.5% of Mo;

at most 0.5% of V;

the complement being iron.

The tensile strength and the impact strength obtained with that steelare compatible with the envisaged applications, but the temperingproperties and the elevated temperature hardness of the carburized layerare insufficient for the applications cited above and for workingtemperatures of up to 220° C.

U.S. Pat. No. 4,157,258, granted to T. V. Philip and R. L. Vedder onJun. 5^(th) 1979, describes a steel with the following chemicalcomposition as a percentage by weight:

0.06%-0.16% of C;

0.2%-0.7% of Mn;

0.5%-1.5% of Si;

0.5%-1.5% of Cr;

1.5%-3% of Ni;

1%-4% of Cu;

2.5%-4% of Mo;

≦0.4% of V;

≦0.05% of P;

≦0.05% of S;

≦0.03% of N;

≦0.25% of Al;

≦0.25% of Nb;

≦0.25% of Ti;

≦0.25% of Zr;

≦0.25% of Ca

the complement being iron.

The compromise between tensile strength and impact strength for thatsteel is good. The carburized layer allows a tempering temperature of upto about 260° C. The maximum working temperature is about 230° C.

However, none of the prior art carburizing steel compositions can allowa tempering temperature for the carburized layer of up to 350° C. to beused, nor do they provide good elevated temperature hardness for workingtemperatures of up to 280° C. while preserving satisfactory corecharacteristics.

There is currently a need for such steels in a number of fields. As anexample, regarding the manufacture of gear parts for helicopters,regulations require that a helicopter must be capable of functioning forthirty minutes after losing oil from its transmission following anincident. That requirement assumes that the materials used tomanufacture the gears have been tempered at a minimum temperature ofabout 280° C.

In the field of engines, designers tend to increase the workingtemperature of engine parts and its connected equipment such asgearboxes, in order to increase yields and/or to simplify heatextraction circuits. Depending on the location of the parts in thisequipment, working temperatures can reach 280° C., imposing a minimumtempering temperature of 330° C. to guarantee that properties are stableduring use.

The present invention aims to provide a carburizing steel compositionthat has all of the characteristics mentioned above.

In a first aspect, the invention provides a carburizing steelcomposition comprising, by weight:

0.06% to 0.18% of C;

0.5% to 1.5% of Si;

0.2% to 1.5% of Cr;

1% to 3.5% of Ni;

1.1% to 3.5% of Mo;

and, if appropriate:

at most 1.6% of Mn; and/or

at most 0.4% of V; and/or

at most 2% of Cu; and/or

at most 4% of Co;

the complement being constituted by iron and residual impurities;

the weight contents of Ni, Mn, Cu, Co, Cr, Mo and V in said composition,expressed by weight, satisfying the following relationships:

2.5 ≦Ni+Mn+1.5Cu+0.5Co≦5  (1)

 2.4≦Cr+Mo+V≦3.7  (2).

Preferably, the sulfur content is limited to 0.010% and the phosphorouscontent is limited to 0.020% by weight, for applications in the upperpart of the range, but higher contents are acceptable for otherapplications, provided that they do not cause a reduction in theductility, toughness and fatigue strength properties of the steel.

The amount of elements such as aluminum, cerium, titanium, zirconium,calcium or niobium, which act either to deoxidize or to refine grainsize, is preferably limited to 0.1% by weight each.

Regarding the principal elements of the composition, in general it hasbeen shown that low carbon, silicon, molybdenum, chromium, and vanadiumcontents, and high manganese, nickel, cobalt, and copper contents canimprove the ductility and toughness of the steel.

In contrast, high carbon, silicon, molybdenum, chromium, and vanadiumcontents and low manganese, nickel, cobalt, and copper contents canimprove the tempering strength of the steel.

The essential role of carbon is to contribute to producing hardness,tensile strength, and hardenability. For carbon contents of less than0.06% by weight, the hardness and tensile strength obtained in the coreof carburized and treated parts are insufficient.

In practice, the desired minimum tensile strength is about 1000 MPa,i.e., about 320 VH (Vickers hardness). The higher the carbon content,the greater the hardness, tensile strength and hardenability but, at thesame time, the impact strength and toughness decrease. For this reason,the carbon content is limited to a maximum of 0.18% by weight.

The most important range for the compromise between tensile strength andtoughness is 0.09%-0.16% by weight of carbon. However, the ranges0.06%-0.12% and 0.12%-0.18% are also of interest for applicationsrequiring different core hardnesses.

Silicon provides a major contribution to the tempering strength of thissteel and its minimum content is 0.5% by weight. In order to avoid theformation of delta ferrite and to retain sufficient toughness, thesilicon content is limited to a maximum of 1.5% by weight. The optimumrange is 0.7%-1.3% by weight, but the range 1.3%-1.5% is also ofinterest.

Chromium contributes to core hardenability and to good temperingstrength of the carburized layer, and its minimum content is 0.2% byweight. To avoid embrittlement of the carburized layer by an excess ofinterlaced carbides, the chromium content must be limited to a maximumof 1.5% by weight. The optimum range is 0.5%-1.2%, but ranges of0.2%-0.8% and 0.8%-1.5% are also of interest.

The role of molybdenum is identical to that of chromium, and it can keepthe elevated temperature hardness high, in particular by formingintragranular carbides in the carburized layer. Its minimum content is1.1% by weight. However, its embrittling effect on this steel limits itsmaximum content to 3.5% by weight. The optimum range is 1.5%-2.5%, butranges of 1.1%-2.3% and 2.3%-3.5% are also of interest.

Vanadium contributes to limiting enlargement of the grain during thecarburizing cycles and treatment cycles used. Because of its embrittlingeffect and its influence on ferrite formation, its content must belimited to a maximum value of 0.4% by weight. The optimum range is0.15%-0.35%, but ranges of 0.05%-0.25% and 0.25%-0.4% are also ofinterest.

Manganese, nickel and copper are gamma-forming elements necessary forequilibrating the chemical composition, avoiding ferrite formation andlimiting the temperature of the α/γ transformation points. They alsoprovide a major contribution to increasing hardenability, impactstrength and toughness but in too high a content, they deteriorate thetempering strength, the elevated temperature hardness and the wearresistance and increase the quantity of residual austenite in thecarburized layer.

For these reasons, the manganese content is limited to a maximum of 1.6%by weight. The optimum range is 0.2%-0.7% by weight, but the range0.7%-1.5% is also of interest. Similarly, the nickel content is limitedto the range 1%-3.5% by weight, the optimum range is 2%-3%, but theranges 1%-2% and 2%-3.5% are also of interest. Finally, copper islimited to a maximum of 2% by weight, the optimum range is 0.3%-1.1% butthe range 1.1%-2% can also be of interest.

Cobalt contributes to the tempering strength of the steel and can reducethe AC point. Its effect is substantially the same for low contents.Large quantities of this gamma-forming element stabilizes the residualaustenite in the carburized layer. The maximum limit is 4% by weight;contents of less than 1.5% by weight are recommended.

In a second aspect, the invention provides a process for producingcarburized and treated parts comprising the following operations:

a. constituting a charge for producing a composition in accordance withthe present invention, as described above;

b. melting said charge in an arc furnace;

c. re-heating and thermomechanical transformation of the ingot;

d. homogenizing heat treatment of the structure and refinement of thegrain;

e. carburizing; and

f. final heat treatment.

The steel of the invention can be obtained using conventional productiontechniques but, to obtain the best results as regards impact strength,toughness and fatigue strength, it is recommended that consumableelectrode remelting is carried out, either with a slurry (ESR) or underreduced pressure (VAR), following arc furnace melting.

To further enhance this performance, it is also possible to carry out afirst melting step by induction under reduced pressure (VIM) and tocontinue with consumable electrode remelting.

The ingots obtained by one of the above methods undergo re-heating attemperatures of about 1100° C. to homogenize the structure, followed bythermomechanical transformations aimed at endowing the product producedfrom this alloy with a sufficient forging ratio or 3 or more (step c) ofthe process of the invention). Lower forging ratios can be used,however, for large parts. Conventional processes, such as rolling,forging, drop forging or drawing, are used for these thermomechanicaltransformations.

A number of implementations can be envisaged regarding step d) of theprocess of the invention. The transformed products can simply besoftened at a temperature below the critical point (AC₁), or tempered ata temperature that is above the critical temperature (AC₁), assuming asufficiently slow onset of cooling.

When the best possible characteristics are required, it is preferable,however, to carry out normalization from a temperature above thecritical point (AC₃), followed by air cooling and softening tempering ata temperature below the critical point (AC₁).

By way of indication, the (AC₁) critical point temperature is generallyin the range from 700° C. to 800° C., while the (AC₃) critical pointtemperature is generally in the range from 900° C. to 980° C.

Carburizing, step e) of the process of the invention, can be carried outusing conventional means, the carburizing cycle being defined by theskilled person depending on the desired hardening depth, in conventionalmanner. A low pressure process can in particular be used.

Regarding step f), the final heat treatment of the part, a variety ofimplementations can be envisaged. It is possible to move directly fromthe carburizing temperature to the austenitization temperature, then toquench the parts, but it is preferable to allow the parts to cool toambient temperature after carburizing, then to re-heat to theaustenitization temperature, above the critical point (AC₃) beforequenching them. By way of indication, the austenitization temperaturerange is 900° C.-1050° C.

The best characteristics of tensile strength, impact strength, coretoughness and superficial hardness of the carburized layer are obtainedby carrying out an oil quench after austenitization, but a goodcompromise between these same characteristics can be achieved bycarrying out a gas quench which has the advantage of reducingdeformation of the parts during this operation and thus minimizingsubsequent machining.

In order to obtain maximum hardness for the carburized layer, and forimpact strength and toughness of the sub-layer, it is preferable totemper at the lowest possible temperature compatible with the workingtemperature. More particularly, a difference of 50° C. between thetempering temperature and the working temperature is preferred, thetempering temperature possibly being up to 350° C.

When producing this steel in large quantities, a continuous castingtechnique can be employed to reduce production costs, but a reduction inductility, impact strength and toughness in particular must be expected.

In a third aspect, the invention provides carburized and treated partsformed from the carburizing steel of the invention which, at ambienttemperature, has a core hardness of close to 320 VH to 460 VH, an ISO Vimpact strength of at least 50 Joules, and more particularly 70 to 150Joules, a toughness of close to 100 MPam, a superficial carburized layerhardness of close to 750 VH, and which, at 250° C., has a superficialcarburized layer hardness of close to 650 VH. These parts canadvantageously be produced using the production process of theinvention, but also using any other process selected as a function ofthe final application.

The following examples demonstrate that a combination of the elementscarbon, manganese, silicon, chromium, nickel, molybdenum, vanadium,copper and cobalt, in the proportions by weight indicated above, resultsin a steel with, simultaneously, excellent hardness, tensile strength,impact strength, impact strength transition and core toughnesscharacteristics, associated with excellent tempering strength andexcellent carburized layer elevated temperature hardness up to workingtemperatures of 280° C.

EXAMPLES

The symbols used have the following meanings:

R_(m)=maximum strength;

R_(p0.2)=conventional yield strength at 0.2% deformation;

E_(5d)=elongation in % over a 5 d base (d=sample diameter);

Z=reduction in area;

VH=Vickers hardness;

RCH=Rockwell hardness;

KV=energy at break, V-notch pendulum impact test.

The examples are supplemented by the figures in the accompanyingdrawings, in which:

FIG. 1 shows the variation in microhardness as a function of depth fortwo samples, the preparation of which is described in Example 1;

FIG. 2 shows the variation in microhardness as a function of depth fortwo samples, the preparation of which is described in Example 2;

FIG. 3 shows the variation in microhardness as a function of depth fortwo samples, the preparation of which is described in Example 3;

FIG. 4 shows the variation in microhardness as a function of depth fortwo samples, the preparation of which is described in Example 4;

FIG. 5 shows the variation in microhardness as a function of depth fortwo samples, the preparation of which is described in Example 5;

FIG. 6 shows the variation in microhardness as a function of depth fortwo samples, the preparation of which is described in Example 6;

FIG. 7 shows the variation in microhardness as a function of depth forthree samples, the preparation of which is described in Example 8.

Example No 1

A 35 kg ingot was produced with the chemical composition shown below, asa percentage by weight, in accordance with the present invention:

C 0.15% Si 1.11% Mn 0.43% Cr 0.92% Ni 2.51% Mo 1.96% V 0.28%

The remainder was constituted by iron and residual impurities.

This ingot was produced by arc melting, then homogenizing at hightemperature to produce a uniform structure, then it was forged. Theforged products were slowly oven cooled. They were normalized todissolve the carbides, homogenize the austenitic structure and refinethe grain.

Bars in accordance with the invention were austenitized at 940° C., oilquenched, cooled in a cryogenic vessel regulated to −75° C. thentempered at a temperature of 250° C.

The mechanical characteristics obtained are shown in the followingtable:

R_(m) R_(p0.2) E_(5d) Z KV (MPa) (MPa) (%) (%) (J) 1427 1101 13.5 60 69

Other samples of this steel were carburized using a low pressure processat a temperature of about 900° C. for 8 hours, then austenitized at 940°C., cooled in a cryogenic vessel regulated to −75° C. and finallytempered at temperatures in the range 150° C. to 350° C.

The superficial hardness of the carburized layer and the core hardnessobtained for different tempering temperatures are shown in the followingtable:

Tempering temperature (° C.) 150 200 250 300 350 Surface hardness, VH800 752 751 735 720 Core hardness, VH 443 438 437 436 437

Hardness measurements were also carried out on polished sections todetermine the hardness gradient in the carburized layer. FIG. 1 showsthe results obtained for tempering temperatures of 150° C. and 350° C.

Example No 2

A 35 kg ingot was produced with the chemical composition shown below, asa percentage by weight, in accordance with the present invention:

C 0.146% Si  1.12% Mn    1% Cr  0.92% Ni  1.54% Mo  1.97% V 0.284%

The remainder was constituted by iron and residual impurities.

This ingot was produced by arc melting, then homogenizing at hightemperature to produce a uniform structure, then it was forged. Theforged products were slowly oven cooled. They were normalized todissolve the carbides, homogenize the austenitic structure and refinethe grain.

Bars resulting from these treatments were austenitized at 940° C., oilquenched, cooled in a cryogenic vessel regulated to −75° C. thentempered at a temperature of 250° C.

The mechanical characteristics obtained are shown in the followingtable:

R_(m) R_(p0.2) E_(5d) Z KV (MPa) (MPa) (%) (%) (J) 1415 1081 13.4 57 51

Other samples of this steel were carburized using a low pressure processat a temperature of about 900° C. for 8 hours, then austenitized at 940°C., cooled in a cryogenic vessel regulated to −75° C. and finallytempered at temperatures in the range 150° C. to 350° C.

The superficial hardness of the carburized layer and the core hardnessobtained for different tempering temperatures are shown in the followingtable:

Tempering temperature (° C.) 150 200 250 300 350 Surface hardness, VH835 748 750 734 722 Core hardness, VH 441 436 435 437 433

Hardness measurements were also carried out on polished sections todetermine the hardness gradient in the carburized layer. FIG. 2 showsthe results obtained for tempering temperatures of 150° C. and 350° C.

Example No 3

A 35 kg ingot was produced with the chemical composition shown below, asa percentage by weight, in accordance with the present invention:

C 0.14% Si 1.49% Mn 0.98% Cr 0.914%  Ni 1.53% Mo 1.99% V 0.284%  Cu0.801% 

The remainder was constituted by iron and residual impurities.

This ingot was produced by arc melting, then homogenizing at hightemperature to produce a uniform structure, then it was forged. Theforged products were slowly oven cooled. They were normalized todissolve the carbides, homogenize the austenitic structure and refinethe grain.

Bars in accordance with the invention were austenitized at 940° C., oilquenched, cooled in a cryogenic vessel regulated to −75° C. thentempered at a temperature of 250° C.

The mechanical characteristics obtained are shown in the followingtable:

R_(m) R_(p0.2) E_(5d) Z KV (MPa) (MPa) (%) (%) (J) 1440 1136 13.2 57 66

Other samples of this steel were carburized using a low pressure processat a temperature of about 900° C. for 8 hours, then austenitized at 940°C., cooled in a cryogenic vessel regulated to −75° C. and finallytempered at temperatures in the range 150° C. to 350° C.

The superficial hardness of the carburized layer and the core hardnessobtained for different tempering temperatures are shown in the followingtable:

Tempering temperature (° C.) 150 200 250 300 350 Surface hardness, VH784 740 740 718 712 Core hardness, VH 451 440 432 447 438

Hardness measurements were also carried out on polished sections todetermine the hardness gradient in the carburized layer. FIG. 3 showsthe results obtained for tempering temperatures of 150° C. and 350° C.

Example No 4

A 35 kg ingot was produced with the chemical composition shown below, asa percentage by weight, in accordance with the present invention:

C 0.11% Si 0.52% Mn 0.49% Cr 0.99% Ni 1.23% Mo 1.96% Co 3.96%

The remainder was constituted by iron and residual impurities.

This ingot was produced by arc melting, then homogenizing at hightemperature to produce a uniform structure, then it was forged. Theforged products were slowly oven cooled. They were normalized todissolve the carbides, homogenize the austenitic structure and refinethe grain.

Bars resulting from these treatments were austenitized at 940° C., oilquenched, cooled in a cryogenic vessel regulated to −75° C. thentempered at a temperature of 250° C.

The mechanical characteristics obtained are shown in the followingtable:

R_(m) R_(p0.2) E_(5d) Z KV (MPa) (MPa) (%) (%) (J) 1045 801 17.5 76 113

Other samples of this steel were carburized using a low pressure processat a temperature of about 900° C. for 8 hours, then austenitized at 940°C., cooled in a cryogenic vessel regulated to −75° C. and finallytempered at temperatures in the range 150° C. to 350° C.

The superficial hardness of the carburized layer and the core hardnessobtained for different tempering temperatures are shown in the followingtable:

Tempering temperature (° C.) 150 200 250 300 350 Surface hardness, VH880 786 749 780 715 Core hardness, VH 371 381 374 374 367

Hardness measurements were also carried out on polished sections todetermine the hardness gradient in the carburized layer. FIG. 4 showsthe results obtained for tempering temperatures of 150° C. and 350° C.

Example No 5

A 35 kg ingot was produced with the chemical composition shown below, asa percentage by weight, in accordance with the present invention:

C 0.12% Si 0.52% Mn 0.47% Cr 0.54% Ni 1.05% Mo   3% V 0.01%

The remainder was constituted by iron and residual impurities.

This ingot was produced by arc melting, then homogenizing at hightemperature to produce a uniform structure, then it was forged. Theforged products were slowly oven cooled. They were normalized todissolve the carbides, homogenize the austenitic structure and refinethe grain.

Bars resulting from these treatments were austenitized at 960° C., oilquenched, cooled in a cryogenic vessel regulated to −75° C. thentempered at a temperature of 250° C.

The mechanical characteristics obtained are shown in the followingtable:

R_(m) R_(p0.2) E_(5d) Z KV (MPa) (MPa) (%) (%) (J) 1149 879 13.6 72 110

Other samples of this steel were carburized using a low pressure processat a temperature of about 900° C. for 8 hours, then austenitized at 960°C., cooled in a cryogenic vessel regulated to −75° C. and finallytempered at temperatures in the range 150° C. to 350° C.

The superficial hardness of the carburized layer and the core hardnessobtained for different tempering temperatures are shown in the followingtable:

Tempering temperature (° C.) 150 200 250 300 350 Surface hardness, VH864 770 716 705 680 Core hardness, VH 440 434 432 423 423

Hardness measurements were also carried out on polished sections todetermine the hardness gradient in the carburized layer. FIG. 5 showsthe results obtained for tempering temperatures of 150° C. and 300° C.

Example No 6

A 35 kg ingot was produced with the chemical composition shown below, asa percentage by weight, in accordance with the present invention:

C 0.12% Si 0.71% Mn 1.57% Cr 1.02% Ni 1.01% Mo 2.02% V 0.01%

The remainder was constituted by iron and residual impurities.

This ingot was produced by arc melting, then homogenizing at hightemperature to produce a uniform structure, then it was forged. Theforged products were slowly oven cooled. They were normalized todissolve the carbides, homogenize the austenitic structure and refinethe grain.

Bars resulting from these treatments were austenitized at 960° C., oilquenched, cooled in a cryogenic vessel regulated to −75° C. thentempered at a temperature of 250° C.

The mechanical characteristics obtained are shown in the followingtable:

R_(m) R_(p 0.2) E_(5d) Z KV (MPa) (MPa) (%) (%) (J) 1258 1009 12.3 71120

Other samples of this steel were carburized using a low pressure processat a temperature of about 900° C. for 8 hours, then austenitized at 960°C., cooled in a cryogenic vessel regulated to −75° C. and finallytempered at temperatures in the range 150° C. to 350° C.

The superficial hardness of the carburized layer and the core hardnessobtained for different tempering temperatures are shown in the followingtable:

Tempering temperature (° C.) 150 200 250 300 350 Surface hardness, VH828 779 754 730 702 Core hardness, VH 441 438 438 439 439

Hardness measurements were also carried out on polished sections todetermine the hardness gradient in the carburized layer. FIG. 6 showsthe results obtained for tempering temperatures of 150° C. and 300° C.Example No 7

A 1000 kg ingot was produced with the chemical composition shown below,as a percentage by weight, in accordance with the present invention:

C 0.14% Si 1.12% Mn 0.44% Cr 0.95% Ni 2.52% Mo 1.93% V 0.27% Cu 0.88%

The remainder was constituted by iron and residual impurities.

This ingot was produced by vacuum induction melting (VIM), then byconsumable electrode remelting, and then re-heated at high temperatureto homogenize the structure. It was then rolled to produce 90 mmdiameter cylindrical rods. These rods underwent a normalizationtreatment to dissolve the carbides, homogenize the austenitic structureand refine the grain.

Samples from these rods were carburized using a low pressure process ata temperature of about 900° C. for 8 hours; samples for characterizingthe core properties underwent an identical thermal cycle but in aneutral atmosphere so that the chemical composition was not modified.

All of the samples were austenitized at 940° C., oil quenched, cooled ina cryogenic vessel regulated to −75° C. then tempered at a temperatureof 300° C.

The mechanical characteristics obtained are shown in the followingtable:

Tempering temperature R_(m) R_(p 0.2) E_(5d) Z KV (° C.) (MPa) (MPa) (%)(%) (J) 300 1430 1111 13 59 75

The test carried out in accordance with American standard ASTM E 399-90on a 20 mm thick CT type sample resulted in a toughness K_(Q) of 107MPam.

The development of the superficial hardness of the carburized layer as afunction of the tempering temperature is shown in the table below:

Tempering temperature (° C.) 150 200 250 300 350 Hardness, VH 802 751745 735 706

The table below shows the development of the superficial hardness of thecarburized layer as a function of the test temperature, using a samplethat had been tempered at 300° C.

Test temperature (° C.) 300 250 200 150 20 Hardness, RCH 57 58 59 60 61

Example 8 (Comparative)

Similar samples were machined in a 16NiCrMo13 steel and carburized underthe same conditions as those described for Example 7.

The samples were then austenitized at 825° C. and oil quenched.

Hardness measurements were carried out on polished sections to determinethe hardness gradient of the carburized layer. FIG. 7 shows the resultsobtained for tempering temperatures of 150° C., 200° C. and 300° C.

The eight preceding examples show firstly that the steels of theinvention represent an excellent compromise between the characteristicsof tensile strength, impact strength and toughness and, secondly, thatthe carburized layer has a high tempering strength and high elevatedtemperature hardness values that are substantially higher than thoseobtained with traditional carburizing steels.

Clearly, the implementations described above are given purely by way ofindication and are in no way limiting, and many modifications canreadily be made by the skilled person without departing from the spiritand scope of the invention.

What is claimed is:
 1. In a carburized steel composition formed from acarburizing steel composition comprising, by weight: 0.06% to 0.18% ofC; 0.5% to 1.5% of Si; 0.2% to 1.5% of Cr; 1% to 3.5% of Ni; 1.1% to3.5% of Mo; 0.37% to 2% Cu; and: at most 1.6% of Mn; and/or at most 0.4%of V; and/or at most 4% of Co; balanced by iron and residual impurities;the improvement characterized by the amounts of Ni, Mn, Cu, Co, Cr, Moand V in said carburizing steel composition, expressed by weight,satisfying the following relationships: 2.5≦Ni+Mn+1.5Cu+0.5Co≦5  (1)2.4≦Cr+Mo+V≦3.7  (2); and characterized in that said carburized steelcomposition allows to maintain a superficial hardness of the carburizedlayer of 650 to 780 Vickers hardness at a temperature above 250° C.
 2. Acarburizing steel composition according to claim 1 comprising, byweight: 0.09% to 0.16% of C; 0.7% to 1.3% of Si; 0.5% to 1.2% of Cr; 2%to 3% of Ni; 1.5% to 2.5% of Mo; 0.2% to 0.7% of Mn; 0.15% to 0.35% ofV; 0.3% to 1.1% of Cu; and: at most 1.5% of Co; balanced by iron andresidual impurities; the amounts of Ni, Mn, Cu, Co, Cr, Mo and V in saidcomposition, expressed by weight, satisfying the followingrelationships: 2.5≦Ni+Mn+1.5Cu+0.5Co≦5  (1) 2.4≦Cr+Mo+V≦3.7  (2).
 3. Acarburizing steel composition according to claim 1, further comprisingat most 0.020% by weight of P and at most 0.010% of weight of S.
 4. Acarburizing steel composition according to claim 3, further containingat most 0.1% by weight of each element Al, Ce, Ti, Zr, Ca, Nb.
 5. Aprocess for producing carburized and treated parts, comprising thefollowing operations: a. constituting a charge for producing a chemicalcomposition according to any one of claims 1 to 4; b. melting saidcharge in an arc furnace; c. re-heating and thermomechanicaltransformation of the ingot; d. homogenizing heat treatment of thestructure and refinement of the grain; e. carburizing; and f. final heattreatment.
 6. A production process according to claim 5, in which arcfurnace melting (step b)) is followed by consumable electrode remelting.7. A production process according to claim 6, in which arc furnacemelting (step b)) is carried out by reduced pressure induction.
 8. Aproduction process according to, claim 5 in which step d) comprisesnormalization at a temperature above the critical point AC₃, air coolingand softening tempering at a temperature below the critical point AC₁.9. A production process according to claim 5, in which step e) iscarried out using a low pressure process.
 10. A production processaccording to claim 5, in which step f) comprises cooling to ambienttemperature then re-heating to 900° C.-1050° C., oil or gas tempering,and tempering at temperatures of up to 350° C.
 11. A steel part with acomposition according to any one of claims 1 to
 4. 12. A steel partcharacterized in that it is obtained by the process of claim 5.